Modified barium titanate ceramic materials and capacitor



MODIFIED BARIUM TITANATE CERAMIC MATERIALS AND CAPACITOR- 2 Sheets-Sheet1 Filed Sept. 12, 1962 no 3 w OM U A 3 m ON G T 1 3 3 9 m w w o N m V NI- O1- w 3 ON+ 3 HONVH'J LNVLSNOQ DIELLDBIHIQ .LNEDBEld INVENTORS THOMASI. PROKOPOWICZ y HAROLD I. GELLER W MM THEIR ATTORNEYS Jan. 25, 1966 T.l. PROKOPOWICZ ETAL 3,231,799

MODIFIED BARIUM 'TITANATE CERAMIC MATERIALS AND CAPACITOR 2 Sheets-Sheet2 Filed Sept. 12, 1962 /m U N HU// NH/ FIGURE 2 THOMAS I. PROKQPOWICZHAROLD GELLER INVENTORS.

COW/700%, a/mb W THEIR ATTORNEYS United States Patent 3,231,799 MODIFIEDBARIUM TITANATE CERAMIC MATERIALS AND CAPACITOR Thomas I. Prokopowicz,North Adams, Mass., and Harold I. Geller, Nashua, N.H., assignors toSprague Electric Company, North Adams, Mass, a corporation ofMassachusetts Filed Sept. 12, 1962, Ser. No. 223,105 8 Claims. (Cl.317-458) This invention relates to ceramic dielectric materials having ahigh dielectric constant which remains comparatively uniform over abroad range of temperatures.

Various titanates, exemplified by barium titanate, have proven quitevaluable for use in forming ceramic bodies of extremely high dielectricconstants. To these materials prior investigators have added variousmate-rials in an effort to improve their electrical properties.Considerable interest has centered on additives which are intended tocontrol within narrow limits the temperature coefificient of dielectricconstant. In one instance it has been taught to add minor amounts ofsodium or potassium niobate (columbate) for this purpose. Subsequentexperimentation has confirmed the efiicacy of these materials for theintended purpose but have also brought to, light a problem inherent inthe use of sodium or potassium niobate.

In the course of firing BaTiO -NaNbO ceramics, it was observed thatweight losses during firing were somewhat greater than that generallyobserved during firing fine-grained BaTiO ceramics. It was determinedthat the weight losses were due to sodium evaporation. Barium titanateis usually fired within the temperature range of 2200-2500 F. Since thesublimation temperature of Na O is about 2327 F. this accounts for thesubstantial amount of sodium oxide lost during firing. The magnitude ofthe loss will vary depending upon the temperature, kiln drafts, thepartial pressure of sodium in the kiln atmosphere, etc. Experience withpotassium oxide, which vaporizes at a considerably lower temperature,has shown that under the same conditions about twice the amount is lostduring firing.

This problem of sodium and potassium evaporation results in a lack ofuniformity and uncertainty in the formation of titanate ceramicsmodified in the above manner. It is necessary to maintain a definiteratio, within narrow limits, between the constituents of the ceramicmaterial in order to obtain a low eoefficient of dielectric constant. Itwould be virtually impossible to compensate for the sodium or potassiumloss by the addition of an excess of sodium or potassium niobate becauseof the many variables involved.

It is an object of the present invention to provide novel ceramicmaterials having comparatively stable electrical properties over a widerange of temperatures.

Still another object of this invention is to provide a ceramic materialthe constitution of which remains substantially constant during firing.

Yet another object of this invention is a novel capacitor having atemperature stable dielectric constant.

These and other objects of the instant invention will become apparentfrom the following description and drawing in which:

FIGURE 1 is a plot of the percentage change in dielectric constant oftwo of these novel ceramic compositions with respect to temperature.

FIGURE 215 a side view in section of a ceramic-capacitor employing thedielectric of the present invention.

The objects of the invention have been achieved by the addition of aneffective amount of the oxides of lithium and niobium; iron and niobium;or lithium, iron and niobium to a barium titanate material. Tantalumoxide (Ta- 0 may be substituted for Nb O in whole or in part.

Lithium oxide (Li O) has a melting point in excess of 3090 F., ironoxide (Fe O has a melting point of 2849 F., niobium oxide has a meltingpoint of 27 68 F. and tantalum oxide has a decomposition point of about2678 F., hence there is no significant additive loss during the firingof the subject ceramics.

The instant invention is illustrated by the following examples.

Example I 13.65 grams BaTiO powder, 0.3988. gram Nb O powder and 0.0240gram Fe O powder are placed into a l00-ml. wide-neck polyethylene bottlefilled /3 with 0.4 inch ceramic milling spheres and containing 40 ml. ofdistilled water. The bottle with its contents is then rotated at 70 rpmfor four hours until an intimate mixture of the powders in water isproduced. The resulting slurry, after removing the milling spheres, isfiltered under vacuum and the resulting cake is dried at C. While thedry filter cake is at 90 C., 6 wt. percent of a paraflfin binder isintimately blended therewith. The blend is forced through a 40-meshscreen to produce granules suitable for pressing. The granulated powderis pressed in a steel die between steel punches at 20,000 p.s.i toproduce discs having a diameter of 0.52 inch and a thickness of 0.04inch. The discs are set on a stabilized zirconia slab which is placedinto a cool electric furnace. The temperature is raised over a period of5 hours to 2320 F. and held there for 1 hour to accomplish sintering ofthe ceramic discs. The furnace is then permitted tocool to roomtemperature. Electrodes are applied to opposing faces of the discs inthe form of a silver paint which on firing to 1300 F. firmly bondsitself to the ceramic.

Ceramic capacitor discs formed in the described manner have a dielectricconstant of 2150 at 25 C. which does not vary more than 6% over thetemperature range of 55 C. to C. (see curve A of the drawing). At 25 C.the dissipation factor is 1.1%. The discs contained 97.0 wt. percentBaTiO and 3.0 wt. percent Nb O and Fe O The mole ratio of Nb O to Fe Ois 10:1.

FIGURE 2 illustrates a capacitor of the foregoing example. The capacitor10 comprises a'ceramic dielectric disc 11 having metal electrodes 12 and13 fired thereon.

Example II 15.92 grams BaTiO powder, 0.2791 gram Nb O powder, and 0.2070gram LiNbO powder are processed according to the procedure in Example Iwith the exception that sintering is accomplished at a temperature of2370 F. for 1 hour.

Ceramic capacitor discs formed in this manner have a dielectric constantof 2000 at 25 C. which does not vary more than 7% over the temperaturerange of 55 C. to 125 C. (see curve B of the drawing). At 25 C. thedissipation factor is 0.85%. The discs contained 97.04 wt. percent BaTiOand 2.96 wt. percent of Nb O and LiNbO The mole ratio of Nb O to Li O is2.5: 1.

The compositions contemplated comprise a major proportion of BaTiO forexample from 80 to 99%, and from about 1 to 8 weight percent based onthe total composition of a member of the group consisting of Nb O andF6203; Nb O and T3205 and F6203; Ta2 O5 and LiTaO and LiTaO wherein themole ratio of niobium oxide and tantalum oxide to iron oxide is from2.5:1 to 25:1 and the mole ratio of niobium oxide and tantalum oxide tolithium oxide is from 1:1 to 10: 1.

In order to insure uniformity of composition the firing temperature mustbe kept below the volatilization temperature of the constituent oxidesand yet not so low that efiective firing will not be obtained. Ingeneral, the firing temperature will fall within the range 2200 F. to2600 F. By volatilization temperature is meant that temperature at whichthe oxide is'converted into a vapor or gas.

In order to impart special characteristics to the instant compositionsthe titanates, stannates and zirconates of strontium, calcium, lead andmagnesium may be substituted either individually or in combination forpart of the BaTiO The electrodes contemplated are any of the materialscommonly employed in ceramic capacitors. This invention is not limitedto the particular process employed as long as the firing temperaturedoes not exceed the indicated maximum. Many modifications of the presentinvention are possible in the light of the above disclosures.

What is claimed is:

1. A dielectric ceramic composition consisting essentially of a firedcombination of a major proportion of barium titanate and from 1 to 8weight percent of a member of the group consisting of Nb O and Fe O Nb Oand T3205 and F6203; T3205 and LiTaO and LiTaO wherein the mole ratio ofniobium oxide and tantalum oxide to iron oxide is from 2.5 :1 to 25:1and the mole ratio of niobium oxide and tantalum oxide to lithium oxideis from 1:1 to 1, said composition having been fired at a temperaturebelow the volatilization temperature of the constituent oxides.

2. The composition of claim 1 containing Nb O and PC203- 3. Thecomposition of claim 1 containing Nb O and LiN-bO 4. The composition ofclaim 1 containing LiNbO 5. The composition of claim 1 containing Ta 0and F6203.

6. The composition of claim 1 containing Ta O and Lil 303.

7. The composition of claim 1 containing LiTaO 8. An electricalcapicator comprising, the fired composition of claim 1 having counterelectrodes afiixed thereto.

References Cited by the Examiner UNITED STATES PATENTS 11/1954 Osh-ry106-39 6/1961 Miller 10646 FOREIGN PATENTS 755,860 8/1956 Great Britain.

TOBIAS E. LEVOW, Primary Examiner. JOHN P. WILDMAN, Examiner.

1. A DIELECTRIC CERAMIC COMPOSITION CONSISTING ESSENTIALLY OF A FIRED COMBINATION OF A MAJOR PROPORTION OF BARIUM TITANATE AND FROM 1 TO 8 WEIGHT PERCENT OF A MEMBER OF THE GROUP CONSISTING OF NB2O5 AND FE2O3; NB2O5 AND LINBO3; LINBO3; TA2O5 AND FE2O3; TA2O5 AND LITAO3; AND LITAO3 WHEREIN THE MOLE RATIO OF NIOBIUM OXIDE AND TANTALUM OXIDE TO IRON OXIDE IS FROM 2.5:1 TO 25:1 AND THE MOLE RATION OF NIOBIUM OXIDE AND TANTALUM OXIDE TO LITHIUM OXIDE IS FROM 1:1 TO 10:1, SAID COMPOSITION HAVING BEEN FIRED AT A TEMPERATURE BELOW THE VOLATILIZATION TEMPERATURE OF THE CONSTITUENT OXIDES.
 8. AN ELECTRICAL CAPICATOR COMPRISING, THE FIRED COMPOSITION OF CLAIM 1 HAVING COUNTER ELECTRODES AFFIXED THERETO. 